| Summary: | Knowledge of the life history of the basking shark, Cetorhinus maximus is limited by the inability to observe the species beyond the 'basking' behaviour it exhibits during the summer months. The locations occupied by this species are uncertain for the majority of the year and as such they have remained a species shrouded with mystery. This non-invasive study of the thermal niche of Cetorhinus maximus in situ describes the clustered distribution of Cetorhinus maximus and evidence of strong associations between Cetorhinus maximus occurrence and the presence of a seasonal front. A total of 78.9 hours of observation were achieved during five seasons of survey (1996, 1997 and 2000-2002) off Plymouth south west England. The temperature range of the local population of Cetorhinus maximus was 10.2-14.7°C, indicating that Cetorhinus maximus did not utilise the full temperature range of 10-16.1°C generally available in the area. The majority (69%) of Cetorhinus maximus were sighted in June. The mean sea surface temperature during these observations was 13.2°C. Behavioural response to fine scale temperature variation was studied to identify the existence and thermal boundaries of preferred temperature. The distance of travel varied with different temperature during a 53-minute fine scale track, suggesting temperature related behaviour that may be the thermoregulation of metabolic rate. Thus, temperature and frontal water are important factors in the distribution of Cetorhinus maximus and specific temperature characteristics may be important for energetics and survival. The information gained will aid in delimiting the thermal habitat of basking sharks for determination of their possible winter distributions, opening up the possibility to study the year-round life history of the species. The Marine Biological Association of the United Kingdom
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